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6-Kw/Cm2 UVC Laser Threshold in Optically Pumped Lasers

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6-Kw/Cm2 UVC Laser Threshold in Optically Pumped Lasers Applied Physics Express LETTERS Related content - Improved carrier injection and confinement 2 in InGaN light-emitting diodes containing 6 kW/cm UVC laser threshold in optically pumped GaN/AlGaN/GaN triangular barriers Li-Wen Cheng, Jian Ma, Chang-Rui Cao et lasers achieved by controlling point defect al. - Optoelectronic device physics and formation technology of nitride semiconductors from the UV to the terahertz Theodore D Moustakas and Roberto To cite this article: Ronny Kirste et al 2018 Appl. Phys. Express 11 082101 Paiella - AlGaN-based laser diodes for the short- wavelength ultraviolet region Harumasa Yoshida, Masakazu Kuwabara, Yoji Yamashita et al. View the article online for updates and enhancements. This content was downloaded from IP address 152.7.255.201 on 15/03/2019 at 14:34 Applied Physics Express 11, 082101 (2018) https://doi.org/10.7567/APEX.11.082101 6kW/cm2 UVC laser threshold in optically pumped lasers achieved by controlling point defect formation Ronny Kirste1,2, Qiang Guo1, J. Houston Dycus1, Alexander Franke1, Seiji Mita1,2, Biplab Sarkar1, Pramod Reddy1,2, James M. LeBeau1, Ramon Collazo1, and Zlatko Sitar1,2 1Department of Materials Science and Engineering, North Carolina State University, Raleigh, NC 27695-7919, U.S.A. 2Adroit Materials, 2054 Kildaire Farm Road, Suite 205, Cary, NC 27518, U.S.A. Received June 22, 2018; accepted July 10, 2018; published online July 27, 2018 Optically pumped lasing from AlGaN/AlN multiple quantum wells grown on single-crystalline AlN substrates with lasing thresholds as low as 6kW/cm2 is demonstrated via the reduction of unintentional point defects in the active region and waveguide, which reduces the non-radiative recombination by 2 orders of magnitude. A higher lasing threshold of 11 kW/cm2 is observed for AlGaN barriers, owing to the reduced localization of electrons and holes in the wells. It is shown that for electrically injected UVC laser diodes, AlGaN barriers are essential. © 2018 The Japan Society of Applied Physics lectrically injected AlGaN-based UVC laser diodes are widely desired for application in spectroscopy, E non-line-of-sight communication, and biosensing.1,2) Thus far, optically pumped lasing in the UVC range has been demonstrated by several groups using both sapphire and AlN substrates.3–9) The achieved laser thresholds range from approximately 50 kW=cm2 to several MW=cm2. The dif- ferences in the laser threshold mainly arise from high point defect concentration due to unfavorable growth conditions, as well as high dislocation density in the case of growth on non- native substrates.10–12) Additionally, the design of the multi- ple quantum well (MQW) structure influences the perform- ance of the optically pumped lasers.13) Understanding and controlling all these parameters to achieve the lowest possible laser threshold and highest possible internal quantum effi- ciency (IQE) is crucial for achieving electrically injected = lasing. Among others, the performance of MQWs is impacted Fig. 1. STEM image of an Al0.55Ga0.45N Al0.65Ga0.35N MQW structure fi by (1) the Al content in the wells and barriers; (2) the with an EBL. All wells and barriers are well-de ned and of homogenous composition. thickness of the wells and barriers; (3) the number of QWs; and (4) the quality of the MQW structure, with regard to the homogeneity and interfacial roughness.14) However, despite many demonstrations of optically pumped lasing in the UV microscopy (STEM) via the Si-stacking procedure.18) STEM range, little is known about the impact of the MQW design imaging was performed using an FEI G2 60–300 kV Titan parameters on the laser threshold. operated at 200 kV with a 19.6 mrad convergence angle and In this study, 6 kW=cm2 threshold UVC lasing is demon- a 77 mrad collection semi-angle. A typical STEM image of strated via a reduction of the point defect concentration in the a sample with an Al content of 65% in the barriers is shown waveguide and in the MQW. The distinction between the in Fig. 1. All the wells and barriers are homogenous, and design of optically pumped and electrically driven laser struc- the interfaces are sharp and well-defined. For the depicted tures, as they must satisfy different requirements, is discussed. sample, the Al content in the barriers and waveguide is the Optically pumped lasers were grown via metalorganic same (65%). Band-structure potential fluctuations due to chemical vapor deposition on single-crystalline physical vapor inhomogeneous Al incorporation, as previously observed transport grown AlN substrates.3,15–17) First, a homoepitaxial in AlGaN MQWs grown on SiC and sapphire, are never AlN layer of approximately 200 nm thickness was grown. observed for structures grown on AlN substrates with proper Next, a 150-nm-thick AlGaN waveguide with an Al content surface supersaturation control.19–22) of 65% was deposited, followed by a 3× AlGaN=AlGaN After the growth, all samples were thinned and backside- MQW (2.5 nm=2.5 nm). The Al content in the wells for all polished to allow for reliable cleaving. The cleaved laser the investigated samples was 55% for a targeted emission of bars were mounted for photoluminescence (PL) and opti- approximately 265 nm, and the Al content in the barriers cally pumped laser measurements.23) An ArF (λ = 193 nm, was varied from 60 to 100%. The structure was finished by Ephoton = 6.4 eV) laser was used as an excitation source. The a 5-nm-thick AlN layer that was similar to the electron- beam intensity was reduced to a reasonable pumping power blocking layer (EBL) in electrically injected UV lasers and using several neutral-density filters and then focused onto served as a protection layer in the optically pumped lasers. the samples. The excitation power was measured using a Details regarding the growth can be found elsewhere.11) Coherent Powermax power meter. For optically pumped To assess the structural properties of the MQW structures, lasing, light was collected from the cleaved facets of the samples were prepared for scanning transmission electron structures using an optical fiber. Light dispersion and detec- 082101-1 © 2018 The Japan Society of Applied Physics Appl. Phys. Express 11, 082101 (2018) R. Kirste et al. wavelength (nm) (a) 280 270 260 45k sample A (a) PL, ArF laser (b) 40k sample B sample A 35k 30k (b) 25k 20k 15k 10k (c) intensity (arb.units) 5k 300 K (arb.units) intensity 2 K 0 1101004.6 4.8 power density (kW/cm2) energy (eV) Fig. 2. (a) UV light emission intensity of optically pumped lasers as a function of the excitation power density for a sample with AlN barriers, where the waveguide was grown under high (sample A) and low (sample B) Fig. 3. Schematics of conduction-band structures of three samples with supersaturation. It is found that a lower point defect concentration different barrier compositions: (a) barriers with a higher Al content than the (sample A) reduces the laser threshold by approximately 1 order of waveguide, (b) barriers with the same Al content as the waveguide, and magnitude. (b) Low-temperature (2 K) and room-temperature PL spectra of (c) barriers with a lower Al content than the waveguide. an AlGaN=AlN MQW (sample A). The room-temperature IQE of the MQW of 95% at a carrier concentration of 1 × 1018 cm−3 is estimated according to the temperature-dependent PL data. improvement over previously reported values.11) In contrast, sample B showed a significantly more drastic drop of the tion was performed using a 0.75-m single monochromator light intensity under the same illumination condition (not and a cooled charge-coupled device camera. shown). At 1 × 1018 cm−3 (50 kW=cm2), this sample exhib- The effect of the growth conditions on the internal quantum ited only 2=3 of the IQE of sample A, which agrees well with efficiency (IQE) has previously been discussed,11) showing previous results.11) Both the IQE and laser threshold meas- that an increase in supersaturation via higher V=III ratios urements confirm that point defects have a significant impact led to higher IQEs for a given carrier concentration owing on the performance of MQWs and therefore UV light- to a decrease in the non-radiative recombination. This dem- emitting diodes and lasers. It is noted that the impact of the onstrated that in addition to dislocations, point defects were point defects on the laser threshold is more pronounced than another major factor that strongly influenced the optical the impact on the IQE. It is argued that this is because point quality of AlGaN-based UV lasers. Therefore, to reduce the defects not only impact the free carriers and generation laser threshold of optically pumped UV lasers, further reduc- of light via radiative recombination but also cause optical tion of point defects in the MQW and the AlGaN waveguide loss=modal loss via absorption of the laser light waves. This was pursued. Figure 2(a) compares the laser threshold of has recently been investigated experimentally and theoret- an optically pumped laser where the waveguide and MQW ically for UV laser devices.26) Finally, it should be pointed were grown under V=III ratios of 1,300 (sample A) and 400 out that a reduction of the point defect concentration can (sample B) by varying the ammonia flow rate. This represents be achieved using tools besides the V=III ratio, including high and low supersaturation, respectively. Increasing the supersaturation (temperature, carrier gas, pressure, etc.) or supersaturation by increasing the V=III ratio is expected to external measures such as Fermi-level control.24,27) reduce the point defect concentration and the non-radiative On the basis of the low-point defect density sample A, recombination in sample A.11,24,25) a series of MQWs with varying Al content in the barriers was Both laser structures emitted light at 267 nm.
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